1. Field of the Invention
The present invention relates to an electrode of an in-line type color picture tube electron gun and, more particularly, to an electrode suitable for improving the focus characteristic on the periphery of a screen.
2. Description of the Prior Art
An example of conventional electron guns for improving the focus characteristic at the peripheral parts of the screen of a color picture tube is disclosed in Japanese Pat. Laid-Open No. 157936/1984. This electron gun has a structure in which a slit recess is provided on the periphery of an electron beam pass aperture of a second grid electrode, the electrode being so designed as to have a thickness larger in the vertical deflection direction and smaller in the horizontal deflection direction, so that the divergent angle of the electron beam is reduced in the vertical deflection direction. The thickness of the electrode is set at 0.45 to 1.0 times the diameter of the electron beam passage aperture at the portion at which the recess is to be formed and at 0.2 to 0.4 times the diameter of the electron beam passage aperture at the recessed portion.
This prior art has no problem in the color picture tube having a screen size of less than 20", but if the color picture ray tube has a screen of 20" or more, e.g., a super large-sized screen of 33", it is impossible to obtain the highly accurate focus characteristic.
The above-described recess, e.g., a recess 3 shown in FIGS. 5(a) and 5(b), are generally formed by coining, as disclosed in, for example, Japanese Pat. Publication No. 4550/1965. In FIGS. 5(a) and 5(b), the reference numeral 4 represents an annular projection.
When an electrode 1 shown in FIGS. 5(a) and 5(b) is fabricated, an excess metal absorb hole 5 is first provided at the portion which corresponds to an electron beam pass aperture 2, as shown in FIGS. 6(a) and 6(b), in order to reduce the coining force, as disclosed in Japanese Pat. Publication No. 4550/1965. A recess 3 is then formed on a recess forming surface 6 on the periphery of the excess metal absorb hole 5 by coining, as shown in FIGS. 7(a) and 7(b), so that the excess metal absorb hole 5 is reduced in size, as indicated by the reference numeral 7. Thereafter, the excess metal absorb hole 7 is formed into a predetermined electron beam pass aperture 2.
In the above-described prior art, the excess metal inevitably produced when coining the recess 3 flows in the direction of both a plastic flow 8 toward the absorb hole contracting direction and a plastic flow 9 toward a projection 4, as shown in FIG. 7(b). The plastic flow 9 toward the projection 4 is absorbed in the transformed shape, as indicated by the broken line 10, but causes an irregular roundness, in other words, a defect around the entire periphery of the opening 11 of the recess 3. The size of the defect is irregular on the periphery of the recess 3, which fact leads to nonuniformity in the configurative accuracy. In particular, the configuration of the recess 3 in the vertical deflection direction (direction of y) in the vicinity of the electron beam pass aperture 2 matters in producing a step on the recess 3 in the vicinity of the aperture 3 in FIG. 5.
As described above, since the prior art involves the problem of nonuniformity in the configurative accuracy of the recess in the vertical deflection direction, it is impossible to obtain uniformity in the focus characteristic on the color picture tube.
When an electrode 1 shown in FIGS. 10(a) and 10(b) is fabricated, an excess metal absorb hole 5 is first provided at the portion which corresponds to an electron beam pass aperture 2, as shown in FIGS. 11(a) and 11(b), in order to reduce the coining force, as disclosed in Japanese Pat. Publication No. 4550/1965. A recess 3 is then formed on a recess forming surface 6 on the periphery of the excess metal absorb hole 5 by coining, as shown in FIGS. 12(a) and 12(b), so that the excess metal absorb hole 5 is reduced in size, as indicated by the reference numeral 7. Thereafter, the excess metal absorb hole 7 is formed into a predetermined electron beam pass aperture 2.
In the above-described prior art, the excess metal inevitably produced when coining the recess 3 flows in the directions of both a plastic flow 8 toward the absorb hole contracting direction and a plastic flow 9 toward a projection 4, as shown in FIG. 12(b). The plastic flow 9 toward the projection 4 is absorbed in the transformed shape, as indicated by the broken line 10, but causes an irregular roundness, in other words, a defect around the entire periphery of the opening 11 of the recess 3. The size of the defect is irregular on the periphery of the recess 3, which fact leads to nonuniformity in the configurative accuracy. In particular, the configuration of the recess 3 in the vertical deflection direction (direction of y) in the vicinity of the electron beam pass aperture 2 matters in producing a step on the recess 3 in the vicinity of the aperture 3 in FIG. 10.
As described above, since the prior art involves the problem of nonuniformity in the configurative accuracy of the recess in the vertical deflection direction, it is impossible to obtain uniformity in the focus characteristic on the color picture tube.